Somewhere out there a perfect player exists for every video game. That means there exists a player who can beat your level perfectly with the information they are given. This is the main point of the perfect player philosophy. Basically you want to design levels that the perfect player would be able to complete with no explanation to them.
This means that you need to design levels that are intuitive to your playerbase. You do not want to have levels that are confusing to the player. The player needs to always have a sense of direction and meaning while playing your game. This is where the perfect player philosophy shines. Because if you design levels intuitively there will only be perfect players. Which means that players will never get stuck in your game and they will be able to enjoy it. Intuitive level design is the best type of level design because you are able to teach mechanics through visual cues. For example if you wanted to teach a player how to jump, you would make a small gap and have the player jump across it. You don’t need to tell the player to jump since they will jump intuitively to get across the gap. This is the perfect player philosophy since it’s better to show your player how to do something rather than tell them. This is because players like to figure out problems on their own as it makes them feel more accomplished. However when designing a level you have to make sure it’s a fair level. For example you don’t want the player to perform frame perfect jumps of a ledge since that is very unfair. If you have unfair sections in your game your player will feel cheated. So remember to keep your levels fair and to design them intuitively. Because there exists a perfect player who will figure out your level just by looking at visual cues.